Diminutive of "my bad", the enunciation of which saves neither the enunciator nor the listener any time of mind or eardrum, though which does indeed spare the enunciator the torment of touching the tip of his tongue to the front upper roof of the mouth as when one does actually say "bad".

Yo 'sup?! I lament the laze and malaise of the muthafucka who can't bring himself to say "my bad" but instead must employ "my b" in its stead, even though there is no contractive benefit gained by the effort, muthafuckas!